Conventionally, as one of backlight devices used in liquid crystal display devices, there is known a backlight device called an edge-light type or a side-light type. In the edge-light type backlight device, in general, light-emitting elements such as LEDs (light-emitting diodes) or CCFLs (Cold Cathode Fluorescent Lamps) are disposed near a side surface or two side surfaces which are opposite surfaces or four side surfaces of a light guide plate fabricated using a transparent resin, and surface emission toward a liquid crystal panel is performed based on light exiting from the light-emitting elements and entering the light guide plate. Note that in the following a device using LEDs as light-emitting elements will be described as an example.
FIG. 13 is a cross-sectional view showing a configuration of an edge of a conventional edge-light type backlight device. The backlight device is composed of LEDs 40; a chassis (hereinafter, referred to as the side chassis) 51 that supports aboard (not shown) having the LEDs 40 mounted thereon; a light guide plate 20 for allowing light emitted from the LEDs 40 to exit toward a liquid crystal panel in a planar manner; a reflection sheet 30 for allowing light traveling toward a back surface side within the light guide plate 20 to be reflected thereby; optical sheets 80 for increasing the efficiency of light irradiated to the liquid crystal panel; and a chassis (hereinafter, referred to as the top chassis) 52 disposed on top of the optical sheets 80. Note that, as shown in FIG. 13, the optical sheets 80 are disposed on a light-emitting surface side of the light guide plate 20 and the reflection sheet 30 is disposed on a back surface side of the light guide plate 20.
In the backlight device shown in FIG. 13, the optical sheets 80 include a reflection type polarizer 81, a prism sheet 82, and a diffuser 83. The diffuser 83 diffuses light to make the light uniform. The prism sheet 82 collects light in a light traveling direction so as to obtain a large amount of components in a direction perpendicular to the liquid crystal panel. The reflection type polarizer 81 allows some light (e.g., linearly polarized light that oscillates in a specific direction) to be transmitted therethrough and allows other light (e.g., linearly polarized light that oscillates in a direction perpendicular to the specific direction) to be reflected thereby. The light having been transmitted through the reflection type polarizer 81 enters a polarizer which is provided on a backlight device side, out of polarizers which are provided on both sides of the liquid crystal panel.
In a configuration such as that described above, light emitted from the LEDs 40 enters the light guide plate 20 directly or after being reflected by the reflection sheet 30. The light having entered the light guide plate 20 propagates through the light guide plate 20 while being reflected within the light guide plate 20 and exits to the light-emitting surface side through the optical sheets 80.
Now, the reason that the optical sheets 80 are made shorter in a left-right direction in FIG. 13 than the light guide plate 20 (the edges of the optical sheets 80 are present in the vicinity of the midpoint between a center side edge surface of the top chassis 52 and an edge surface (edge) of the light guide plate 20) will be described. If, as shown in FIG. 14, the length of the optical sheets 80 is made equal to the length of the light guide plate 20, then light exiting from an LED 40 in a direction of an arrow indicated by reference character 71 propagates through the optical sheets 80 as shown by an arrow indicated by reference character 72, resulting in a cause of light leakage from an edge portion of a display region. Hence, in order that such light can be absorbed by the top chassis 52, the optical sheets 80 are made shorter than the light guide plate 20, as shown in FIG. 13.
Meanwhile, narrowing of a picture-frame of a liquid crystal display device has been promoted in recent years. Hence, narrowing of a picture-frame of a backlight device has also been promoted. FIG. 15 is a cross-sectional view showing a configuration of an edge of a conventional edge-light type backlight device with a narrowed picture-frame. Now, taking a look at the distance in the left-right direction in FIGS. 13 and 15 between the center side edge surface of the top chassis 52 and the edge surface of the light guide plate 20, a distance L9 in the configuration shown in FIG. 15 is shorter than a distance L8 in the configuration shown in FIG. 13. As an example, in a 40-inch liquid crystal panel, the distance L9 is on the order of 5 mm. Due to the distance between the center side edge surface of the top chassis 52 and the edge surface of the light guide plate 20 being thus short, unlike the configuration shown in FIG. 13, the length of optical sheets 90 is made equal to the length of the light guide plate 20 in the left-right direction in FIG. 15. The reason for this is as follows. During use of the backlight device, with light emission of LEDs 40, the optical sheets 90 expand by heat. Hence, the size of the optical sheets 90 is designed taking into account a certain level of tolerance. Therefore, the optical sheets 90 move (are displaced) within the tolerance range rather than being fixed within a chassis. Under such a presumption, if the configuration is such that the distance from the center side edge surface of the top chassis 52 to the edges of the optical sheets 90 is made short as shown in FIG. 16A, then when, for example, a liquid crystal display device is transported, the optical sheets 90 may jump out of the chassis by vibration, as shown in FIG. 16B. Hence, in a backlight device with a narrowed picture-frame seen in recent years, in order that the optical sheets 90 are held within the chassis, the distance from a position corresponding to the center side edge surface of the top chassis 52 on the optical sheets 90 to the edges of the optical sheets 90 is set so as to have a sufficient length covering (the optical sheets 90) with the top chassis 52. For example, as shown in FIG. 15, the length of the optical sheets 90 is made equal to the length of the light guide plate 20.
However, according to the configuration shown in FIG. 15, as described above, there are concerns about the occurrence of light leakage from an edge portion of a display region. Specifically, light exiting from an LED 40 in a direction of an arrow indicated by reference character 73 in FIG. 17 propagates through the optical sheets 90 as shown by an arrow indicated by reference character 74, resulting in a cause of light leakage. Hence, there is proposed a backlight device having a configuration in which, as shown in FIG. 18, black printing 60 for absorbing light which is a cause of light leakage is provided in a part of the front surface or back surface of the optical sheets 90. According to the backlight device having the configuration shown in FIG. 18, light that exits from LEDs 40 and enters the optical sheets 90 without entering the light guide plate 20 is absorbed by the black printing 60. Hence, in the backlight device with a narrowed picture-frame, the occurrence of light leakage is suppressed.
Note that in relation to the invention of this matter, Japanese Patent Application Laid-Open No. 2004-71167 discloses an invention of a planar light source device that suppresses the occurrence of emission lines and display unevenness caused by light leakage, by sticking a light-proof tape onto all or a part of an edge portion on the light-emitting surface side of a light guide plate.